1. Field of the Invention
The present invention relates to a process for wearing a film comprising a polymer capable of forming an optically anisotropic melt phase.
The film treated by the process of the present invention has the excellent resistance to heat and chemicals and the electrical properties inherent to polymers capable of forming an optically anisotropic melt phase, as well as a small dimensional change ratio when heated, resistance to intra-layer delamination, excellent resistance to folding, a moderate thermal expansion coefficient, and high strength and elongation. This film is, therefore, useful as a material for insulting tapes, packaging films and laminates for flexible printed wiring boards, multilayer thin film wiring boards, damping materials, and the like.
2. Description of the Prior Art
Polymers capable of forming an optically anisotropic melt phase (hereinafter sometimes referred to as "liquid crystal polymers") exhibit excellent resistance to heat and chemicals and excellent electrical properties (e.g., insulation property, dielectric property, etc.). These polymers, therefore, have attracted attention in various technical fields as useful materials for films.
Films made from liquid crystal polymers tend to have highly anisotropic mechanical properties due to high orientability during the melt extrusion molding of the polymers. Thus, melt extrusion of a liquid crystal polymer through a T die typically produces a uniaxially oriented film because shear or stress is applied only in the machine direction (hereinafter referred to as "MD"). The uniaxially oriented film has high tensile modulus and mechanical strength in MD, but low corresponding values in a direction perpendicular to MD (transverse direction, hereinafter referred to as "TD"), and hence has the drawback of readily forming tears in MD.
To reduce the anisotropy in mechanical properties, the application of a tubular film process to melt extrusion molding of liquid crystal polymers has been proposed (U.S. Pat. No. 4,333,907 and U.S. Pat. No. 5,053,481). This process involves stretching, or applying stress to the film, both in MD and TD to produce biaxially oriented films, which do not preferentially tear or split in MD. A tubular film process also can produce films in which the mechanical properties in MD and TD are well balanced.
However, uni- or biaxially oriented liquid polymer films obtained by the above melt extrusion processes have poor abrasion resistance, thereby tending, when their surface is rubbed, to form fibrils on the surface. They also have a high intra-layer delamination tendency so that laminates comprising the films and other materials tend to delaminate. Furthermore, these liquid crystal polymer films have internal swain due to stress having been applied during molding, and hence they tend to undergo dimensional changes or deform when heated for processing or other purposes. In addition, liquid crystal polymer films lack flexibility and readily break when folded repeatedly.
In order to improve the abrasion resistance and intra-layer delaminatability, a process of calendering a liquid crystal polymer film at a temperature that does not melt the film (Japanese Patent Application Laid-open No. 62144/1993), and a process of embossing the film under the same condition (Japanese Patent Application Laid-open No. 166323/1992), have been described.
A process for improving the dimensional stability of liquid crystal polymer films when heated, which comprises heating them at a temperature of about 30.degree. C. lower than its melting point or below, has been described in U.S. Pat. No. 4,333,907. It has been reported that the use of this type of heat treatment, at a temperature below the flow initiation temperature, on a film obtained by melt extruding liquid crystal polyester while applying to it a shearing stress in TD, produced a heat treated film having a heat shrinkage at 180.degree. C. of 0.03 to 0.1% (Japanese Patent Application Laid-open No. 23921/1991).
The above process of heat treating liquid crystal polymer films at a temperature below their melting point or flow initiation temperature tends to produce films having insufficient dimensional stability, unless a film produced by a special melt extrusion process is used, as described above. Heat treatment at a temperature below the melting point or flow initiation temperature is not widely usable for the purpose of obtaining a film having excellent dimensional stability.
The melt pressing of pellets of a liquid crystal polymer is a known alternative to melt extrusion as a process for molding liquid crystal polymer films. This process produces unoriented films, since liquid crystal polymers generally are not oriented during melt pressing (U.S. Pat. No. 4,717,624).
The melt pressing of liquid crystal polymer pellets can give a film having some, what better resistance to abrasion and intra-layer delamination as compared with films obtained by melt extrusion, but the film produced by this method has low tensile strength, and the film thickness is hard to control, thereby making this process of low practical applicability.
There is also known a process for producing laminates, which comprises heating a liquid crystal polymer film, which is contacted with a sheet of metal plate or metal foil or sandwiched between 2 sheets of metal plate or metal foil, under pressure, at a temperature at which the liquid crystal polymer either melts or does not melt, to bond them (U.S. Pat. No. 4,717,624 and Japanese Patent Application Laid-open Nos. 252738/1990 and 53739/1992 and 136038/1992 and European Patent Application Publication No. 507,332). However, the laminates obtained by this process are intended to be used as composites for printed wiring boards, vibration damping materials and the like, utilizing their laminate structure. Thus, there have been available no reports paying attention to the properties of the liquid crystal polymer layer itself formed in such laminates. Furthermore, it is not known to obtain a film by separating the liquid crystal polymer layer from the laminate.
Calendering or embossing liquid crystal polymer films under non-melting conditions does not sufficiently improve the intra-layer delamination and sometimes insufficiently improves their abrasion resistance.